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We have investigated the hypothesis that uridine 5'-diphosphate (UDP)-glucuronyltransferases (UGTs) and beta-glucuronidase are jointly involved in a mechanism for the storage and mobilization of iodothyronine metabolites in liver, kidney, heart and brain. Specifically, we predicted UGT activities to decrease and increase respectively, and beta-glucuronidase activity to increase and decrease respectively in hypo- and hyperthyroidism. To this end we have studied the effects of thyroid status on the activities of different enzymes involved in thyroid hormone metabolism in liver, kidney, heart and brain from adult rats with experimentally induced hypo- and hyperthyroidism. We used whole organ homogenates to determine the specific enzyme activities of phenol- and androsteron-UGT, beta-glucuronidase, as well as iodothyronine deiodinase types I and II. Deiodinase type I activities in liver and kidney were decreased in hypothyroid animals and, in liver only, increased in hyperthyroidism. Deiodinase type II activity was increased in hyperthyroid rat kidney only. Interestingly, in the heart, deiodinase type I-specific activity was increased fourfold, although the increase was not statistically significant. Cardiac deiodinase type I activity was detectable but not sensitive to thyroid status. Hepatic phenol-UGT as well as androsteron-UGT activities were decreased in hypothyroid rats, with specific androsteron-UGT activities two to three orders of magnitude lower than phenol-UGT activities. Both UGT isozymes were well above detection limits in heart, but appeared to be insensitive to thyroid status. In contrast, cardiac beta-glucuronidase activity decreased in hypothyroid tissue, whereas the activity of this enzyme in the other organs investigated did not change significantly.In summary, cardiac beta-glucuronidase, albeit in low levels, and hepatic phenol-UGT activities were responsive only to experimental hypothyroidism. Although a high basal activity of the pleiotropic beta-glucuronidase masking subtle activity changes in response to thyroid status cannot be ruled out, we conclude that hepatic, renal and cardiac UGT and beta-glucuronidase activities are not regulated reciprocally with thyroid status.

We have investigated the potential role of fibroblasts in local thyroid hormone metabolism in neonatal rat heart. Incubation of cardiac fibroblasts with thyroxine (T4) or 3,5,3'-tri-iodothyronine (T3) resulted in the appearance of water-soluble metabolites, whereas incubation of cardiomyocytes under the same conditions did not or did so to a much lesser extent. Time-course studies showed that production is already evident after 1-5 h of exposure and that the process equilibrates after 24-48 h. Analysis of the products revealed both the T4 and the T3 metabolites to be glucuronides. These results were corroborated by the detection of uridine diphosphate (UDP)-glucuronyltransferase activity in cardiac fibroblasts. We found no indication for outer ring deiodination in fibroblasts, cardiomyocytes or heart homogenates. From these results we have concluded that cardiac fibroblasts, but not cardiomyocytes, are able to glucuronidate T4 and T3 and secrete the conjugates. This could play a role in local metabolism, e.g. to protect the heart tissue from high levels of thyroid hormones.